High power,broadband,electron stream-plasma noise source



C. D. LUSTIG July 7, 1970 HIGH POWER, BROADBAND, ELECTRON STREAM-PLASMANOISE SOURCE .3 Sheets-Sheet 1 Filed Sept. 17, 1964 Rmtm: 3336 S2528I'll Q-- HOFLLINSVW 10.2 69 llll' So u INVENTOR. CLAUDE [7. L usr/aATTORNEY C. D. LUSTIG July 7, 1970 HIGH POWER, BROADBAND, ELECTRONSTREAM-PLASMA NOISE SOURCE Filed Sept. 17, 1964 2 Sheets-Sheet 2ISOLATOR our FIG

STARTING PULSER INVENTOR. CLAUDE 0- L us 7/6 PULSER STARTl NG UnitedStates Patent ()ihce 3,519,951 Patented July 7, 1970 3,519,951 HIGHPOWER, BROADBAND, ELECTRON STREAM-PLASMA NOISE SOURCE Claude D. Lustig,Arlington, Mass., assignor to Sperry Rand Corporation, Great Neck, N.Y.,a corporation of Delaware Filed Sept. 17, 1964, Ser. No. 397,123

Int. Cl. H031) 29/00 US. Cl. 331-78 12 Claims This invention relates toa low pressure plasma noise generator, and more particularly to such agenerator that produces a continuous spectrum of noise over a relativelybroad frequency range and at a power level that is uncommonly high fordevices of this type.

Noise sources are useful in performing certain tests on electromagneticwave receiving equipment and also are useful in a military environmentfor various countermeasures purposes. Noise generators that presentlyare commonly employed include resistors generating thermal noise whenheated to elevated temperatures, gas discharges in fluorescent tubes,and various types of microwave tubes that operate saturated on theirinherent noise. The first two of the above-mentioned noise sources arelimited in their power outputs, their maximum power outputs being arounddb and db, respectively, these being less than one-half the db powerlevel obtainable from the device of this invention. The third-mentionednoise source is comparatively expensive to build and ordinarily israther limited in its frequency range.

It therefore is an object of this invention to provide a relatively highpower noise source that produces a continuous output over a relativelyWide frequency range.

A further object of this invention is to provide a relativelyinexpensive noise source having a controllable high output power.

A further object of this invention is to provide a plasma filleddischarge tube that produces a noise signal over a continuous,relatively wide frequency range and at a high power level that may bevaried by an electrical control signal.

Another object of this invention is to provide a low pressure gasdischarge tube operating under such conditions that it produces bands ofrelatively high power noncoherent noise in both of two different wavepolarizations, the bands of noise of the two polarizations beingcombined by suitable transmission line apparatus to produce a continuousbroad frequency spectrum of relatively high power noise.

These and other objects and advantages of the invention, which willbecome obvious from reading the specification and claims below, areachieved on one illustrated embodiment of the invention by providingfirst and second sections of rectangular waveguide each of which supportelectromagnetic waves only in the dominant TE waveguide mode. Thesewaveguide sections are disposed adjacent each other with their broadwalls perpendicular to each other. An elongated gas discharge tubefilled with a noble gas such as neon at a low pressure of the order of10 microns extends transversely through both waveguides, extendingbetween the broad walls of one waveguide and between the narrow walls ofthe other one. When the discharge is established within the gas tube,relatively incoherent oscillations of electromagnetic waves areestablished in each of the waveguides. Oscillations in the twowaveguides are the same to the extent that they both have a comb-likecharacteristic as a function of frequency, but they differ to the extentthat the frequencies of the oscillations are different in the twowaveguides. The oscillations in one waveguide have maxima that areregularly spaced throughout a wide frequency spectrum, and theoscillations in the other waveguide also have maxima that are regularlyspaced throughout the same frequency range, but these maxima fall midwaybetween frequencies of the maxima of oscillations in the firstwaveguide. By cOmbining the outputs of both waveguides a relatively highlevel, continuous broad frequency spectrum of noise will be available atan output terminal. These results may be obtained without the use of amagnetizing field in the discharge. By applying a unidirectionalmagnetic field of variable strength, the oscillations may be set to acertain magnitude, or may be modulated in amplitude.

The present invention will be described by referring to the accompanyingdrawings wherein:

FIG. 1 is an illustration, partly in schematic form, of one embodimentof the present invention;

FIG. 2 is a graph showing several curves that are used in explaining theoperation of the apparatus illustrated in FIG. 1; and

FIGS. 3 and 4 are simplified illustrations of further embodiments of thepresent invention.

Referring now in detail to the drawings, the embodiment of the inventionillustrated in FIG. 1 is comprised of first and second sections ofhollow rectangular waveguides 11 and 12, each adapted to propagateelectromagnetic waves in the dominant TE mode, and both havingapproximately the same physical dimensions so that they propagate waveswithin the same frequency range. As illustrated, waveguides 11 and 12are positioned with their respective broad walls orthogonal to eachother so that the directions of the electric fields of the dominantmodes in the two waveguides also are orthogonal. Waveguides 11 and 12each are provided with a termination 13, 14, respectively, at one end,and the waveguide-to-coaxial line couplers 16 and 17 coupleelectromagnetic waves from the other ends of the waveguides.

An elongated low pressure gas discharge tube 20 extends transverselythrough both waveguides 11 and 12. The longitudinal axis of tube 20 isperpendicular to the direction of polarization of the electric field Eof TE mode electromagnetic waves in waveguide 11, and parallel to thedirection of polarization of the electric field E of TE mode waves inWaveguide 12. Gas discharge tube 20 includes at one end an electronemissive cathode 21, such as an oxide coated nickel mesh cathode, and anelectron collecting anode electrode 22 is positioned at the oppositeend. Discharge tube 20 is made of a nonconducting material such as glassor quartz, and in one embodiment of the invention, had an inner diameterof 9 millimeters and a length of 3 feet, although the length is notcritical. Tube 20 is filled with an ionizable gas at a low pressure. Ihave successfully used neon gas at pressures ranging between 10 and 50microns. I also have successfully used other noble gases such as helium,argon, and krypton at pressures of approximately 80, 5, and 1 microns,respectively. Mixtures of the above gases may be employed as theionizable medium, if desired. Oscillations in mercury and xenon gaseswere of too small a magnitude to be of practical utility as a noisesource. Solenoids 23, 24 and 25 may be disposed about discharge tube 20in the region of waveguides 11 and 12 to establish, when desired, alongitudinally directed unidirectional magnetic field through thecentral portion of the tube.

Cathode 21 is heated by means of a current from battery 26 and thepotential between cathode 21 and anode 22, which was 1000' volts in oneoperating embodiment of the invention, is established by battery 27. Adischarge is initiated within tube 20 by means of starting pulser 28,and the discharge is stabilized by means of a resistor 29 of severalhundred ohms in the biasing circuit. I have successfully operated thedischarge tube with a discharge current of approximately one-quarterampere, although this value is not critical.

As will be explained in more detail below, electromagnetic waves aregenerated in gas tube 20' and propagate within the respective waveguides21 and 22. The waves are coupled from the respective Waveguides throughthe waveguide to coaxial transmission line couplers 16 and 17 and arecoupled through suitable transmission lines 30 and 31 to a T-junction33. Variable attenuators 36 and 37 are disposed in the lines 30 and 31to equalize the power level of the two inputs to T-junction 33. Junction33 combines the two input signals thereto, and the combined signal iscoupled through isolator 38 to a suitable output terminal. It should beunderstood that other appropriate means may be employed for coupling theelectromagnetic waves from the waveguide sections 11 and 12. These othermeans may employ coaxial or rectangular waveguide transmission lines andmay include different types of combining networks.

Upon the establishment of the discharge within discharge tube 20 and theionization of the gas contained therein due to the collision of theelectrons in the electron stream with the gas atoms, well-definedrelatively wide bands of noncoherent oscillations at regularly-spacedfrequency intervals are set up in each one of the waveguides 11 and 12.The oscillations in each waveguide have a comb-like characteristic as afunction of frequency, and the maxima of the various oscillatory bandsare not the same in the two waveguides, but rather, the maxima of theoscillatory bands in one waveguide fall intermediate the maxima of theoscillatory bands in the other waveguide. This is illustrated in FIG. 2,wherein the curve designated E i represents the type of responsedetected in waveguide 11 wherein the axis of discharge tube 20 isperpendicular to the direction of the electric field vector E and thecurve designated E represents the type of response detected in waveguide12 wherein the axis of discharge tube 20 is parallel to the electricfield vector E The maxima of each of the curves are regularly occurringthroughout the frequency spectrum and have a separation of approximately310 megacycles per second when neon gas at a pressure of approximately20 microns is used. When argon gas was used, the frequency separationbetween the maxima was approximately 250 megacycles per second. Afterthe signals E and E from waveguides 11 and 12 have been combined in T-junction 33, the combined signal will be a continuous, relativelyconstant magnitude and relatively incoherent noise signal that iscoupled to the output terminal. As may be seen from FIG. 2, thebandwidth of the combined output signal extends approximately from 2gigacycles cycles) per second to 4 gigacycles per second. The bandwidthof the microwave circuitry employed will of course alfect the bandwidthof the output signal. Ridged Waveguides may be used for the waveguides11 and 12 in order to obtain broad bandwidths in the microwavecircuitry.

I have observed that the spacing between adjacent maxima on each curvedecreases as the internal diameter of the discharge tube increases.Therefore, by choosing the correct diameter for discharge tube 20, themaxima of the two curves E i and E may be adjusted to assure that thecombined curves produce the desired continuous, relatively fiat responsewhich may be used as a broadband noise signal.

The magnitude of the combined output signal was found to be unexpectedlyhigh for devices of this type, being of the order of 10* or 10 watts permegacycle per second, which is approximately 30 db or 40 db higher thanthat provided by commercially available standard noise tubes operatingin the microwave frequency range.

I am at present unable to fully explain the reasons for the types ofoscillations detected in the waveguides 11 and 12, and their behaviorcannot be explained by existing theory and observations of resonantphenomena such as the Tonks-Dattner resonances that are reported, forexample, in Physical Review Letters, 11, 183, 1963 by I.

C. Nickel, I. V. Parker, R. W. Gould. I have obtained noise outputs thatrepresent noise figures greater than 50 db, and this magnitude of outputis too great to be accounted for by single particle electron emissionfrom the plasma. As used above, the term noise figure is a relativefigure based on the noise output of a gas discharge tube at roomtemperature as the reference. Although the exact mechanism by which theoscillatory signals are generated is not known, it is believed that theyare correlated with irregular current bursts in the discharge.

In operating embodiments of the invention the percent ionization of thegases within the discharge tube 20 ranged between .01 percent and .1percent. Although the electrons in this type of discharge are not inthermal equilibrium, a measure of electron temperature that was madewithin the operating discharge tube indicated a mean electrontemperature of approximately 8 to 10 electron volts. These electrontemperatures are to be compared with the mean electron temperature ofapproximately 3.5 electron yolts that was measured when the differentTonks-Dattner resonances were observed, these Tonks-Dattner resonancesbeing of a very much lower magnitude of approximately 10* watts permegacycle per second.

The root means square plasma frequency was approximately at the centerof the emitted band of frequencies. For radiation from '2-4 gigacyclesper second this represents a mean electron density of approximately 10electrons per cubic centimeter, although the electron density may varywithin the approximate range of 10 to 10 electrons per cubic centimeter.In operating embodiments of this invention I have found that thefrequencies of the bands of oscillations varied but slightly withchanges in the electron density, the frequency of the maxima shifting byonly 3 percent when the electron density was doubled. Changes inelectron density did, however, have some effect on the amplitudes of theoscillatory bands. As the electron density was increased, the amplitudesof the higher frequency oscillatory bands increased while the amplitudesof the lower frequency oscillatory bands were diminished.

Further, changes in the gas pressure within tube 20 has negligibleeffect on the frequencies of oscillations so long as the pressures ofhelium, neon, argon, and krypton were below approximately 100, 50, 5,and 1 microns, respectively.

All of the results and effects that have been discussed above areobtainable without a magnetizing field being applied to the ionized gaswithin discharge tube 20, and it is possible that a fully operable noisesource may be constructed without the use of a magnetizing field. I havefound, however, that when a unidirectional magnetic field is appliedalong the longitudinal axis of discharge tube 20 by the solenoids 23,24, and 25, the frequencies of the oscillations are not affected, buttheir magnitudes are affected. Beginning with a low magnetic fieldstrength of several gauss, and with the discharge tube filled with neongas at a pressure of approximately 20 microns, the magnitude of theenvelope of the curves of FIG. 2 will increase to a maximum value at afield strength of approximately 4 gauss, and then will decrease as themagnetic field strength is further increased. Oscillations were nolonger detectable when the applied magnetic field was in the rangebetween 20 and 30 gauss. This affords the possibility of controlling themagnitude of the noise output to a desired value, and also affords thepossibility of modulating the amplitude of the output noise signal bymeans of an electrical modulating signal applied to the solenoids.

Another embodiment of the present invention is illustrated in FIG. 3 inwhich the gas discharge tube 20 and its associated discharge circuitry,as well as the noise signal combining circuitry, are substantiallyidentical to those illustrated in FIG. 1 and are designated by the samereference numerals. In FIG. 3, discharge tube 20 extends transverselythrough the vertical walls 41-42 of square hollow uniconductor waveguidesection 43. Square waveguide section 43 is dimensioned to supportelectromagnetic waves in orthogonally disposed dominant TE modes. Whenthe discharge is initiated within discharge tube 20, oscillations ofboth of the types illustrated in FIG. 2 will be set up in squarewaveguide section 43. The component B will be coupled from waveguidesection 43by coaxial coupler 16, and the component B will be coupledfrom waveguide section 43 by coaxial coupler 17. These two componentswill be balanced in magnitude by the attenuators 36 and 37 and will becombined in T-junction 33 to form the continuous, high power, broadbandnoncoherent noise signal that is coupled through isolator 38 to theoutput terminal. The section of square waveguide 43 will be closed atits left end by a suitable termination, which is not illustrated.

Another embodiment of this invention is illustrated in FIG. 4 in whichthe gas discharge tube 60 includes the conventional electron emissivecathode 61 and collector anode 62, which together with voltage sources26 and 27 and resistor 29 establish a discharge within tube 60.Discharge tube 60 is filled with one of the above-identified ionizablegases under the appropriate conditions to establish the same type ofdischarge and ionization condition as described above. In thisembodiment of the invention, gas discharge tube 60 is tapered indiameter throughout its length so as to form a conically-shaped envelopewhich is made of glass, quartz, or other suitable material.

As mentioned previously, I have determined that the various frequenciesat which oscillations were detected is a function of the diameter of thegas discharge tube. Therefore, by varying the diameter of tube 60throughout an appropriate range of diameters, relatively high poweroscillations may be obtained throughout a substantially continuous broadrange of frequencies, thereby avoiding the comb-like characteristic thatis obtained from a gas tube of constant diameter traversing a singlehollow waveguide. Further, there is no need in this embodiment of theinvention for associated transmission line circuitry to combine twodifferent oscillatory signals as. was necessary in the embodiments ofFIGS. 1 and 3.

In this embodiment of the invention, the relatively continuous spectrumof oscillation is coupled out to coaxial transmission line terminal 50by means of a TEM mode transmission line arrangement comprised of anarrow thin conductive member 65 which is spiraled about the envelope ofgas discharge tube 60 and makes conductive contact with the innerconductor 66 of coaxial transmission line terminal 50. Acomically-shaped conductive member 68 is disposed in spaced relationshipabout gas discharge tube 60 and acts as the ground conductor of the TEMmode transmission line arrangement. Means are provided at the right endof the comically-shaped conductive member 68 to suitably terminate thatend of the device.

In the operation of the device of FIG. 4, the parameters of the gasdischarge tube '60 and its exciting circuitry are adjusted to establishthe previously-mentioned conditions within the tube, and because of thevarying diameter of tube 60 oscillations of relatively incoherentelectromagnetic waves will be produced throughout a substantiallycontinuously frequency spectrum and these oscillations are coupled tothe TEM mode transmission line arrangement comprised of strip conductor65, ground conductor 68 and will propagate to the output terminal 50.

It shoulld be understood that gas discharge tube 60 may be reduced indiameter throughout its length by a series of successively spaceddiscrete steps rather than being uniformly tapered as illustrated inFIG. 4.

Any of the devices illustrated in FIGS. 1, 3 or 4 also may be utilizedas a signal source for supplying oscillations over a relatively narrowfrequency range by providing frequency selective filtering means at theoutput of the waveguide sections, and by omitting the combiningT-junctions in the embodiments of FIGS. 1 and 3. With this type ofarrangement, any desired one of the various maxima of oscillations maybe selected for transmission to a suitable utilization means.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

What is claimed is:

1. A plasma source of oscillations comprising:

an elongated gas container having a transverse dimension which variesalong its length for confining a gas under pressure,

an ionizable gas within said container at a pressure below approximatelymicrons,

means for ionizing said gas to the extent of between .01 to .1 percentand to produce an electron density within said container ranging betweenapproximately 10 and 10 electrons per cubic centimeter,

the mean energy of electrons within the plasma,

as determined by electron temperature measurements, ranging betweenapproximately 810 electron volts, said ionized gas being characterizedby generating oscillations of electromagnetic waves over a relativelywide frequency range, and electromagnetic waves supporting meansdisposed in wave coupling relationship with said ionizable gas forcoupling to said oscillations.

2. The combination claimed in claim 1 and further including:

means for immersing said gas container in a magnetic field.

3. The combination claimed in claim 1 wherein said electromagnetic wavesupporting means is comprised of TEM mode wave propagating means.

4. The combination claimed in claim 1 wherein the gas is selected fromthe group of noble gases that includes helium, neon, argon and krypton.

5. A plasma source of oscillations comprising:

a gas container for confining a gas under pressure,

an ionizable gas within said container, said gas being selected from thegroup of noble gases that includes helium, neon, argon and kryptonmaintained at a pressure below a maximum pressure of approximately 100,50, 5, and 1 micron, respectively,

means for ionizing said gas to the extent of between .01 to .1 percentand to produce an electron density within said container ranging betweenapproximately 10 and 10 electrons per cubic centimeter,

the mean energy of electrons within the plasma, as determined byelectron temperature measurements ranging between approximately 8-10electron volts,

said ionized gas being characterized by generating oscillations ofelectromagnetic waves over a relatively wide frequency range, and

electromagnetic waves supporting means disposed in wave couplingrelationship wtih said ionizable gas for coupling to said oscillations.

6. Apparatus for producing relatively high power, broadband, incoherentoscillations of electromagnetic waves, said apparatus comprising:

an elongated gas discharge tube comprised of a tube of dielectricmaterial having an ionizable gas confined therein,

a section of hollow waveguide adapted to support orthogonally polarizedoscillations of electromagnetic waves within a given frequency range,

said discharge tube extending transversely through said waveguide sothat a discharge within said tube presents an ionized gas within thebounds of said waveguides, and

electromagnetic wave coupling means for coupling to electromagneticwaves in each of said two orthogonal polarizations within saidwaveguide. 7. A high power noise source comprising: first and secondrectangular waveguides having broad and narrow walls and each adapted topropagate electromagnetic waves within a given frequency range in thedominant T E mode,

said waveguides being disposed adjacent each other with their respectivebroad walls orthogonal to each other, an elongated gas discharge tubeextending transversely between the narrow walls of the first waveguideand transversely between the broad walls of the second Waveguide, andmeans for extracting and combining electromagnetic waves from said twowaveguides to produce a composite electromagnetic wave signal. 8. A highpower noise source comprising: a gas discharge tube containing anionizable gas at a pressure below approximately 100 microns, means forestablishing a discharge within said tube to ionize said gas to theextent between .01 to .1 percent,

said discharge tube, with the discharge established therein, generatingelectromagnetic waves that are supportable in two electromagnetic wavemodes, means disposed in electromagnetic wave coupling relationship withsaid discharge tube for supporting oscillations of said generatedelectromagnetic waves in each of said two modes,

the waves of said two modes being characterized by having differentfrequency contents, and means for combining the waves of said two modesto produce a composite signal therefrom having the frequency content ofboth of said modes. 9. A high power noise source comprising: a gasdischarge tube filled with an ionizable gas at a pressure belowapproximately 100 microns, means for establishing a discharge of chargedparticles within said tube to ionize said gas to the extent from .01 to.1 percent,

said discharge tube, with the discharge established therein, generatingelectromagnetic waves having components that are supportable in twoorthogonal electromagnetic wave dominant modes,

electromagnetic wave structure adapted to support electromagnetic wavesin said two modes,

said structure being disposed relative to said tube to receive from saidtube electromagnetic waves in said two modes, and

means for extracting and combining from said structure electromagneticwaves in said two modes.

10. The combination claimed in claim 9 and further including:

means for establishing a unidirectional magnetic field directed alongsaid discharge tube.

11. The combination claimed in claim 9 wherein the gas is selected fromthe group of noble gases that includes helium, neon, argon and krypton.

12. The combination claimed in claim 11 wherein the gas within saidcontainer is one of the named gases at a pressure of 100, 50, 5 and 1micron, respectively.

References Cited UNITED STATES PATENTS 2,706,782 4/1955 Mumford 331782,716,192 8/1955 Johnson 33178 2,745,013 5/1956 Hines 33178 2,855,51410/1958 Skolnik 33178 2,872,581 2/1959 Page 331-78 2,942,204 6/ 1960Poulter 331-78 3,023,374 2/1962 Mumford 33178 2,868,978 1/1959 Kearneyet al 33178 OTHER REFERENCES Gaseous Discharge Super High FrequencyNoise Sources, Proc. of IRE, vol. 39, July-December 1951, pp. 90s 914.

Electrical Noise Generators, Proc. of IRE, vol. 35, No. 9, September1947, pp. 875-879.

RODNEY D. BENNETT, 111., Primary Examiner D. C. KAUFMAN, AssistantExaminer US. Cl. X.R. 3l3183

7. A HIGH POWER NOISE SOURCE COMPRISING: FIRST AND SECOND RECTANGULARWAVEGUIDES HAVING BROAD AND NARROW WALLS AND EACH ADAPTED TO PROPAGATEELECTROMAGNETIC WAVES WITHIN A GIVEN FREQUENCY RANGE IN THE DOMINANTTE10 MODE, SAID WAVEGUIDES BEING DISPOSED ADJACENT EACH OTHER WITH THEIRRESPECTIVE BROAD WALLS ORTHOGONAL TO EACH OTHER, AN ELONGATED GASDISCHARGE TUBE EXTENDING TRANSVERSELY BETWEEN THE NARROW WALLS OF THEFIRST WAVEGUIDE AND TRANSVERSELY BETWEEN THE BROAD WALLS OF THE SECONDWAVEGUIDE, AND MEANS FOR EXTRACTING AND COMBINING ELECTROMAGNETIC WAVESFROM SAID TWO WAVEGUIDES TO PRODUCE A COMPOSITE ELECTROMAGNETICWAVEGUIDE SIGNAL.